JP3473480B2 - Hot-dip galvanized steel sheet excellent in strength and ductility and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet having excellent strength and ductility, which is suitable for use in press forming, bending, etc., such as automobile bodies, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the weight of vehicle bodies of automobiles has been reduced in order to improve fuel efficiency. Therefore, in order to ensure safety, steel sheets for automobiles have a tensile strength (TS) of 490 MPa or more,
High ductility is required to ensure moldability. The surface of the steel sheet is hot-dip galvanized in order to ensure rust prevention. Therefore, automobile steel sheets are strongly required to have high strength, high ductility and hot dip galvanizing property.

As one of methods for increasing the strength of a steel sheet, a method is known in which the structure of the steel sheet has a composite structure in which a hard martensite phase is dispersed in a soft ferrite phase.

A steel sheet having a composite structure of ferrite and martensite has a transformation temperature to austenite (AC ₁ transformation temperature).
From the above, it can be obtained by heating below the transformation end temperature to austenite (AC ₃ transformation temperature) to form a two-phase structure of austenite and ferrite, and then cooling to transform the austenite phase into the martensite phase. At this time,
When the austenite phase is unstable, it transforms into a pearlite phase, so a martensite structure cannot be obtained. Therefore, in order to stabilize the austenite phase and suppress the transformation into the pearlite phase, a method of adding an alloy element is usually adopted.

For example, as a method for producing a high-strength steel sheet having excellent formability of a press-formed product, a steel sheet containing a large amount of Si, P, and Mn added is placed between a plating tank and an alloying furnace in a hot dip galvanizing facility. There is a method of making a steel sheet having a composite structure of a ferrite structure and a martensite structure by maintaining the steel plate at a temperature of the Ms point or higher (see JP-A-55-100935). The steel sheet containing Mn and Cr was heated between the AC ₁ transformation temperature and the AC ₃ transformation temperature, and the temperature was changed to 450 ° C.
It cools to the temperature range of ~ 550 ℃ at the critical cooling rate, and 450 ~
Hot-dip galvanized in the temperature range of 550 ℃, and further 500 ℃
A method has been proposed in which an alloying treatment is performed by heating between the temperature and the AC ₁ transformation temperature, and cooling from that temperature to 300 ° C or less at a critical cooling rate (Japanese Patent Laid-Open No. 55-122821). .

[0006]

The addition of solid solution strengthening elements such as Si, Mn, and P requires a large amount of addition to bring the tensile strength of the steel sheet to 490 MPa or more, resulting in a high cost. In addition, if a large amount of Si, P, etc. is added, the plating property deteriorates,
The plating quality is poor. Further, a method of adding a precipitation strengthening element such as Ti, Nb, or V is known. However, since the ductility is deteriorated, the press formability is remarkably deteriorated.

For example, the invention disclosed in JP-A-55-100935 discloses a large amount of alloying elements (Si: 0.60% or less, Mn:
3.5% or less, P: 0.150% or less, Si + Mn: 2.3% or more), which results in high cost. In addition, the method of controlling the cooling rate according to the alloy content in the hot dip galvanizing equipment (Japanese Patent Laid-Open No. 55-122821) has a high cooling rate when the alloy elements are reduced (for example, when Cr is not added). Therefore, additional equipment for cooling is required.

An object of the present invention is to provide a hot-dip galvanized steel sheet having a low alloy content, a tensile strength of 490 MPa or more and an excellent ductility, and a method for inexpensively producing the same, using an existing hot-dip galvanizing facility. To provide.

[0009]

Means for Solving the Problems The inventors of the present invention have earnestly studied a method for obtaining a steel sheet having both high strength and high ductility, and have obtained the following findings.

(1) Precipitation element (T
i, Nb, V) is added.

(2) If the C content is higher than the precipitation of the carbides of the above elements (Ti, Nb, V), the C dissolved in fine austenite becomes spherical cementite, which reduces the decrease in ductility of the steel sheet. .

(3) In order to improve the press workability of the steel sheet, the product of tensile strength and elongation may be 18000 or more. If the above (1) and (2) are satisfied, the tensile strength of the steel sheet will be 490 MP
A steel sheet having a product of a or more and tensile strength and elongation of 18,000 or more can be obtained.

The present invention has been completed based on the above findings, and its gist resides in the hot-dip galvanized steel sheet shown below and its manufacturing method.

% By weight, C: 0.10 to 0.20%, Si: 0 to
0.2%, Mn: 0.5-1.8%, P: 0.005-0.03%, S: 0.01
% Or less, sol.Al: 0.01 to 0.1%, N: 0.002 to 0.008%,
B: 0 to 0.005 %, Ti: 0.01 to 0.1%, Nb: 0.005 to
0.05%, V: 0.01 to 0.1% of any one kind or two kinds or more, and a balance of Fe and unavoidable impurities in the steel sheet.
A hot-dip galvanized steel sheet having a structure in which carbonitrides of V and spherical cementite are precipitated, and the surface of which is coated with a galvanized layer.

The steel having the above chemical composition is usually hot-rolled and wound at 400 to 700 ° C., then pickled and cold-rolled at a reduction ratio of 40 to 80% to 750 ~ 850 ° C
A method for producing a high-strength hot-dip galvanized steel sheet, which is subjected to normal plating treatment after being subjected to recrystallization annealing in.

The hot-dip galvanized steel sheet of the present invention has a structure in which spherical cementite and carbonitrides of Ti, Nb, and V are precipitated in the matrix of the ferrite phase, and the tensile strength (TS) is 490.
It is a steel plate having a good balance of strength and ductility, since the product (TS x El) of MPa or more and tensile strength (TS) and elongation (El, ductility) is 18000 or more.

The reason for this is not clear so far, but the following reasons can be considered.

In a hot dip coating facility, heating to a temperature range from the AC ₁ transformation point or more to the AC ₃ transformation point or less (recrystallization annealing temperature range) results in a two-phase structure of a ferrite phase and an austenite phase. However, since the steel sheet of the present invention contains a large amount (0.02% or more) of carbonitride, the austenite structure becomes extremely fine during heating, which contributes to increasing the tensile strength. Further, the remaining C (carbon) that has formed carbide dissolves in the fine austenite phase, and spherical cementite is formed by the subsequent cooling. Thereby, the ductility of the steel sheet is less deteriorated and the tensile strength can be increased.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for defining the chemical composition of the hot-dip galvanized steel sheet of the present invention will be described below. In addition,
"%" In the chemical composition of the steel sheet means "% by weight".

C: C combines with Ti, Nb, V, etc. to form a carbide, which precipitates in the steel and imparts a predetermined strength to the steel sheet.
The present invention is characterized in that it is added in an amount more than necessary for forming carbide. The remaining carbon that has formed carbonitrides forms a solid solution in the austenite phase formed during the heating of recrystallization annealing in the hot dip coating facility. When it is subsequently cooled, spherical cementite is formed, which contributes to improvement of ductility and further strength increase. If the C content is less than 0.1%, the tensile strength and ductility cannot be improved. Further, if it exceeds 0.2%, a pearlite phase is precipitated during cooling to deteriorate ductility,
It also deteriorates the weldability. Therefore, the content of C is
It was set to 0.1 to 0.2%.

Si: Si is extremely effective as a strengthening element, and since the deterioration of ductility is small, it is an element to be added in a large amount. However, Si significantly impairs the plating property, so the content is set to 0.2% or less.

Mn: Mn is an element effective as a strengthening element, and is contained by 0.5% or more. However, if it exceeds 1.8%, the press workability is impaired and the cost increases. Therefore, the Mn content is 0.5 to 1.8%.

P: P is an element effective as a strengthening element and is added in an amount of 0.005% or more. On the other hand, if it exceeds 0.03%, the plating property is impaired. Therefore, the P content is 0.005 to 0.03.
%.

S: S is an impurity which inevitably enters from the raw material. However, if the content exceeds 0.01%, the press workability of the steel sheet is impaired. Therefore, the S content is 0.
It was set to 01% or less.

Sol.Al: sol.Al is contained in steel by deoxidizing molten steel at the time of melting with aluminum (Al). This has the effect of binding to N (nitrogen) in the steel to form AlN precipitates and suppressing coarsening of austenite crystal grains. Therefore, it is necessary to contain sol.Al in an amount of 0.01% or more. However, even if it is contained excessively, the effect is saturated, and if it exceeds 0.1%, the press workability is impaired. Therefore, the sol.Al content is set to 0.01 to 0.1%.

N: N has the effect of suppressing the growth of austenite crystal grains during recrystallization annealing. Therefore, 0.002
% Or more. However, if the content exceeds 0.008%, the ductility (elongation) decreases. Therefore, the N content is
0.002 to 0.008%

Ti, Nb and V: Ti, Nb and V have the effects of forming carbonitrides and contributing to the increase in strength, and suppressing the coarsening of the recrystallized structure and the two-phase structure during soaking. In order to exert these effects, Ti, Nb and V
Among them, one kind or two or more kinds are contained. However, if it is contained excessively, the recrystallization temperature rises and good ductility cannot be obtained. Therefore, their contents are in the following ranges.

Ti: 0.01 to 0.1% Nb: 0.005 to 0.05% V: 0.01 to 0.1% B (boron): B may not be contained. But B
Is a carbide in the austenite phase (carbide of Fe and B)
Is stabilized, and during cooling, the cementite is spherically precipitated with the carbide as a nucleus to suppress the precipitation of coarse pearlite. Thereby, the deterioration of the ductility of the steel sheet can be suppressed more effectively. When B is contained, it is desirable to contain 0.0005% or more. However, if it exceeds 0.005%, the effect is saturated. Therefore, the B content is set to 0 to 0.005%.

Next, a method for manufacturing the plated steel sheet of the present invention will be described.

From melting to hot rolling, there is no particular limitation, and a commonly used method may be used. However, in order to refine the structure after hot rolling, the coil winding temperature should be 700 ℃.
The following is preferable. Also, when it becomes less than 400 ° C,
Hard bainite or martensite structure is generated,
Cold rolling property deteriorates.

The cold rolling may be carried out by a commonly used method, but the higher the cold rolling rate is, the better in order to refine the structure after recrystallization annealing. However, if the cold rolling ratio is less than 40%, sufficient recrystallization does not occur in recrystallization annealing. On the other hand, if it exceeds 80%, the rolling load becomes high and troubles such as breakage of the material occur. Therefore, the cold rolling rate is set to 40 to 80%.

In the treatment in the hot dip coating facility, it is necessary to raise the heating temperature to 750 ° C. or higher in order to cause sufficient recrystallization during recrystallization annealing of the steel sheet before plating and to form a two-phase structure. Further, when the temperature exceeds 850 ° C, the crystal grains become coarse. Therefore, the recrystallization annealing temperature of the steel sheet before plating is
It was set to 750 to 850 ° C. For other conditions, the plating treatment that is usually performed may be performed. Since the steel sheet of the present invention has little change in mechanical properties due to the alloying treatment after plating, it may or may not be alloyed.

The hot-dip galvanized steel sheet of the present invention has a high C content and contains one or more of Ti, Nb and V, so that carbides other than Ti, Nb and V are precipitated. C forms a solid solution in austenite refined by recrystallization annealing. C dissolved in the refined austenite precipitates as spherical cementite in the subsequent cooling process.

[0034]

EXAMPLES Example 1 Steels having the chemical compositions shown in Tables 1 and 2 were melted in a converter and slabs (width
1200 mm, thickness 240 mm, length 8 m).

[0035]

[Table 1]

[0036]

[Table 2]

These slabs were heated at 1200 ° C. for 30 minutes, rolled at a finishing temperature of 850 ° C. in a continuous hot rolling facility, and then wound into coils at the temperatures shown in Tables 3 and 4,
A hot rolled steel sheet having a plate thickness of 4 mm was obtained. After pickling these hot rolled steel sheets, cold rolling and hot dip galvanizing were performed. The cold rolling ratio and the recrystallization treatment temperature in the hot dip galvanizing treatment are shown in Tables 3 and 4. The holding time at the recrystallization treatment temperature was 60 seconds.

[0038]

[Table 3]

[0039]

[Table 4]

The plating conditions are as follows: after the above recrystallization treatment, cooling to 500 ° C. at a cooling rate of 10 ° C./sec.
Plating was performed by immersing in a zinc bath of About some steel plates, it alloyed by heating at 500 degreeC. The deposition amount of plating was 50 g / m ^{2 in} all cases.

JIS No. 5 tensile test pieces were sampled from the rolling direction of the plated steel sheet and tested for mechanical properties (yield strength, tensile strength and elongation). The results, and the yield ratio and the product of tensile strength and elongation calculated from them are also shown in Tables 3 and 4.

As is clear from these results, the steel sheets of Test Nos. 1 to 19 of the invention examples had the chemical composition of the steel, the coil winding temperature, the cold rolling rate, and the soaking temperature within the ranges defined by the present invention. , 509-627 MPa tensile strength and 18021-188
The product of tensile strength and elongation of 57 is obtained, and the strength and ductility (press formability) are excellent.

On the other hand, the steel sheet of Test No. 20 of the comparative example has a low tensile strength of 477 MPa because it does not contain Ti, Nb and V in the chemical composition of the steel.

The steel sheets of Test Nos. 21 and 22 have a low C content of 0.09% and 0.06% in the chemical composition of the steel, so that the tensile strength and elongation are low, and the product of tensile strength and elongation is 17069.
And as low as 15432.

The steel sheet of Test No. 23 has a large C content of 0.22% in the chemical composition of the steel, so that the product of tensile strength and elongation is 1623.
As low as 3.

The steel sheet of Test No. 24 had a large Si content of 0.22% in the chemical composition of the steel, and therefore, uneven plating appearance occurred.

The steel sheet of Test No. 25 has a low Mn content of 0.4% in the chemical composition of the steel, and therefore has low tensile strength and elongation, and the product of tensile strength and elongation is as low as 16250.

The steel sheet of Test No. 26 has a high Mn content of 1.9% in the chemical composition of the steel, and therefore has a low elongation of 26.2% and a low product of tensile strength and elongation of 16506.

In the steel sheet of Test No. 27, the P content of the steel chemical composition was as large as 0.035%, so P segregation occurred and uneven plating appearance occurred due to the difference in alloying behavior.

The steel sheet of Test No. 28 has a low S content of 0.015% in the chemical composition of the steel, and therefore has a low elongation, and the product of tensile strength and elongation is as low as 16981.

In the steel sheet of Test No. 29, the Al content in the chemical composition of the steel is as small as 0.005%, so Ti, Nb and V become oxides, which increase the tensile strength and decrease the elongation (26.2%). The product of tensile strength and elongation is as low as 16852.

The steel sheet of Test No. 30 has a large Al content of 0.15% in the chemical composition of the steel, and therefore has a large amount of oxides and solid solution Al (sol.Al), and has a low elongation of 27.4% and a high tensile strength and elongation. Product of 15
As low as 825.

The steel sheet of Test No. 31 has a small N content of 0.0015% in the chemical composition of the steel, so that the precipitation of nitrides such as Ti, Nb and V is small. For this reason, austenite coarsens during recrystallization annealing and pearlite is generated during cooling, which reduces the elongation to 26.1% and the product of tensile strength and elongation.
As low as 15425.

The steel sheet of Test No. 32 has a large N content of 0.0095% in the chemical composition of the steel, so that the amount of solute nitrogen increases and the elongation increases.
The product of tensile strength and elongation is as low as 15892.

The steel sheet of test No. 33 does not recrystallize because the recrystallization temperature rises because the Ti content of the steel chemical composition is as high as 0.14%. Therefore, the elongation is low at 21.8% and the product of tensile strength and elongation is low at 14210.

The steel sheets of Test Nos. 34 and 35 have high Nb and V contents of 0.056% and 0.12% in the chemical composition of the steel, so that the recrystallization temperature rises and does not recrystallize. For this reason, the elongation is low at 19.3% and 22.2%, respectively, and the product of tensile strength and elongation is low at 14532 and 14608.

Steel sheets of test numbers 36 to 50 are examples in which the winding temperature in hot rolling, the cold rolling reduction rate, and the soaking temperature deviate from the ranges defined by the present invention.

Steel sheets having test numbers 36, 37 and 38 were all steels having a chemical composition within the range defined by the present invention. However, since the coiling temperature for hot rolling was lowered to 350 ° C.,
The hardness of the hot rolled steel sheet was high, and material fracture occurred during cold rolling.

The steel sheets of Test Nos. 39, 40 and 41 were all steels having a chemical composition within the range defined by the present invention. However, since the coiling temperature in hot rolling was as high as 720 ° C. The crystal grains become large, the balance between tensile strength and elongation is poor, and the product of tensile strength and elongation is 16399, 16702 and
As low as 16880.

The steel sheets of Test Nos. 42, 43 and 44 were all steels having a chemical composition within the range defined by the present invention. However, since the cold rolling ratio was as low as 35%, the crystal grains were recrystallized during annealing. It becomes large, the balance between tensile strength and elongation is poor,
The product of tensile strength and elongation is low at 15947, 16039 and 15530.

Steel sheets having test numbers 45, 46 and 47 were all steels having a chemical composition within the range defined by the present invention. However, since the cold rolling rate was as high as 85%, the rolling load was large, Material fracture occurred.

For the steel sheets of Test Nos. 48, 49 and 50, steels having a chemical composition within the range defined by the present invention were used. However, since the soaking temperature was as low as 730 ° C., recrystallization did not occur, so The balance between strength and elongation is poor, and the product of tensile strength and elongation is low at 14434, 14963 and 14219.

[0063]

The hot-dip galvanized steel sheet of the present invention is made of Ti, N
Since it has a structure in which carbonitrides b and V and spherical cementite are precipitated in ferrite, the steel sheet has an excellent balance between strength and ductility and excellent press formability. This steel sheet is low in cost because it contains few alloying components, and can be easily manufactured by an existing hot dip coating line.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-11-61269 (JP, A) JP-A-4-314828 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. By weight%, C: 0.10 to 0.20%, Si: 0 to 0.2
%, Mn: 0.5 to 1.8%, P: 0.005 to 0.03%, S: 0.01%
Below, sol.Al: 0.01-0.1%, N: 0.002-0.008%,
B: 0 to 0.005 %, Ti: 0.01 to 0.1%, Nb: 0.005 to
0.05%, V: 0.01 to 0.1% of any one kind or two kinds or more, and a balance of Fe and unavoidable impurities in the steel sheet.
A hot-dip galvanized steel sheet excellent in strength and ductility, which has a structure in which carbonitrides of V and spherical cementite are deposited and whose surface is coated with a galvanized layer. 2. A steel having the chemical composition according to claim 1,
After normal hot rolling and winding at 400-700 ℃, pickling treatment is performed, cold rolling is performed at a reduction rate of 40-80%, and 750
A method for producing a hot-dip galvanized steel sheet having excellent strength and ductility, which comprises performing recrystallization annealing at a temperature of up to 850 ° C and then performing a normal plating treatment.